Synchronous motors, including permanent magnet motors and line-start permanent magnet motors, are typically very efficient. However, synchronous motors have limited capability to pull into synchronism loads that have a high torque or high inertia. Accordingly, the benefits in efficiency gains and energy savings ordinarily associated with synchronous motors are not typically achieved in applications having loads with high inertia and/or high torque characteristics. In the past, an inverter has been used with synchronous motors in such applications to power the motor during starting. However, an inverter adds substantial costs and degrades system efficiency.
To achieve the benefits of efficiency provided by synchronous motors, for instance, permanent magnet line-start motors, a mechanical soft-start coupling may be used as an interface between the synchronous motor and a high inertia and/or high torque load to enable the synchronous motor to bring the load up to or near synchronous speed. The soft-start coupling effectively isolates the synchronous motor from the high inertia and/or high torque load for enough time to enable the synchronous motor to come up to full synchronous speed. The soft-start coupling then brings the load up to or near synchronous speed. In the case of a lockingtype of soft-start coupling, the load speed will eventually be the same as the speed of the synchronous motor. For the case of a soft-start coupling that continues to slip even at steady state, the load speed will only approach but not equal the motor speed. As described below, a synchronous motor with a soft start coupling may be started directly across a polyphase power system and provide a dramatic reduction in noise and mechanical stress during the starting process due to a great reduction in torque ripple transmitted to the driven load. The soft-start coupling enables rapid acceleration of the motor by isolating the load from the motor temporarily until the motor comes up to synchronous speed.